File-Based Encryption

Android 7.0 and later supports file-based encryption (FBE). File-based
encryption allows different files to be encrypted with different keys that can
be unlocked independently.

This article describes how to enable file-based encryption on new devices
and how system applications can be updated to take full advantage of the new
Direct Boot APIs and offer users the best, most secure experience possible.

Warning: File-based encryption cannot
currently be used together with adoptable storage. On devices using
file-based encryption, new storage media (such as an SD card) must be used as
traditional storage.

Direct Boot

File-based encryption enables a new feature introduced in Android 7.0 called Direct
Boot. Direct Boot allows encrypted devices to boot straight to the lock
screen. Previously, on encrypted devices using full-disk
encryption (FDE), users needed to provide credentials before any data could
be accessed, preventing the phone from performing all but the most basic of
operations. For example, alarms could not operate, accessibility services were
unavailable, and phones could not receive calls but were limited to only basic
emergency dialer operations.

With the introduction of file-based encryption (FBE) and new APIs to make
applications aware of encryption, it is possible for these apps to operate
within a limited context. This can happen before users have provided their
credentials while still protecting private user information.

On an FBE-enabled device, each user of the device has two storage locations
available to applications:

Credential Encrypted (CE) storage, which is the default storage location
and only available after the user has unlocked the device.

Device Encrypted (DE) storage, which is a storage location available both
during Direct Boot mode and after the user has unlocked the device.

This separation makes work profiles more secure because it allows more than one
user to be protected at a time as the encryption is no longer based solely on a
boot time password.

The Direct Boot API allows encryption-aware applications to access each of these
areas. There are changes to the application lifecycle to accommodate the need to
notify applications when a user’s CE storage is unlocked in response to
first entering credentials at the lock screen, or in the case of work profile
providing a
work
challenge. Devices running Android 7.0 must support these new APIs and
lifecycles regardless of whether or not they implement FBE. Although, without
FBE, DE and CE storage will always be in the unlocked state.

A complete implementation of file-based encryption on an Ext4 file system is
provided in the Android Open Source Project (AOSP) and needs only be enabled on
devices that meet the requirements. Manufacturers electing to use FBE may wish
to explore ways of optimizing the feature based on the system on chip (SoC)
used.

All the necessary packages in AOSP have been updated to be direct-boot aware.
However, where device manufacturers use customized versions of these apps, they
will want to ensure at a minimum there are direct-boot aware packages providing
the following services:

Telephony Services and Dialer

Input method for entering passwords into the lock screen

Examples and source

Android provides a reference implementation of file-based encryption, in which
vold (system/vold)
provides the functionality for managing storage devices and
volumes on Android. The addition of FBE provides vold with several new commands
to support key management for the CE and DE keys of multiple users. In addition
to the core changes to use the ext4 Encryption
capabilities in kernel many system packages including the lockscreen and the
SystemUI have been modified to support the FBE and Direct Boot features. These
include:

Dependencies

Keymaster
Support with a HAL version 1.0 or 2.0. There is no support for
Keymaster 0.3 as that does not provide that necessary capabilities or assure
sufficient protection for encryption keys.

Keymaster/Keystore and
Gatekeeper must be implemented in a Trusted Execution
Environment (TEE) to provide protection for the DE keys so that an
unauthorized OS (custom OS flashed onto the device) cannot simply request the
DE keys.

Encryption performance in the kernel of at least 50MB/s
using AES XTS to ensure a good user experience.

Hardware Root of Trust and Verified Boot
bound to the keymaster initialisation is required to ensure that Device
Encryption credentials are not accessible by an unauthorized operating
system.

Note: Storage policies are applied to a folder and all of its
subfolders. Manufacturers should limit the contents that go unencrypted to the
OTA folder and the folder that holds the key that decrypts the system. Most
contents should reside in credential-encrypted storage rather than
device-encrypted storage.

Implementation

First and foremost, apps such as alarm clocks, phone, accessibility features
should be made android:directBootAware according to Direct
Boot developer documentation.

Kernel Support

The AOSP implementation of file-based encryption uses the ext4 encryption
features in the Linux 4.4 kernel. The recommended solution is to use a kernel
based on 4.4 or later. Ext4 encryption has also been backported to a 3.10 kernel
in the Android common repositories and for the supported Nexus kernels.

The android-3.10.y branch in the AOSP kernel/common git repository may
provide a good starting point for device manufacturers that want to import this
capability into their own device kernels. However, it is necessary to apply
the most recent patches from the latest stable Linux kernel (currently linux-4.6)
of the ext4 and jbd2 projects. The Nexus device kernels already include many of
these patches.

Note that each of these kernels uses a backport to 3.10. The ext4
and jbd2 drivers from linux 3.18 were transplanted into existing kernels based
on 3.10. Due to interdependencies between parts of the kernel, this backport
breaks support for a number of features that are not used by Nexus devices.
These include:

The ext3 driver, although ext4 can still mount and use ext3 filesystems

Global File Sytem (GFS) Support

ACL support in ext4

In addition to functional support for ext4 encryption, device manufacturers may
also consider implementing cryptographic acceleration to speed up file-based
encryption and improve the user experience.

Enabling file-based encryption

FBE is enabled by adding the flag
fileencryption=contents_encryption_mode[:filenames_encryption_mode]
to the fstab line in the final column for the userdata
partition. contents_encryption_mode parameter defines which
cryptographic algorithm is used for the encryption of file contents and
filenames_encryption_mode for the encryption of filenames.
contents_encryption_mode can be only aes-256-xts.
filenames_encryption_mode has two possible values: aes-256-cts
and aes-256-heh. If filenames_encryption_mode is not specified
then aes-256-cts value is used.

Whilst testing the FBE implementation on a device, it is possible to specify the
following flag:
forcefdeorfbe="<path/to/metadata/partition>"

This sets the device up with FDE but allows conversion to FBE for developers. By
default, this behaves like forceencrypt, putting the device into
FDE mode. However, it will expose a debug option allowing a device to be put
into FBE mode as is the case in the developer preview. It is also possible to
enable FBE from fastboot using this command:

fastboot --wipe-and-use-fbe

This is intended solely for development purposes as a platform for demonstrating
the feature before actual FBE devices are released. This flag may be deprecated
in the future.

Integrating with Keymaster

The generation of keys and management of the kernel keyring is handled by
vold. The AOSP implementation of FBE requires that the device
support Keymaster HAL version 1.0 or later. There is no support for earlier
versions of the Keymaster HAL.

On first boot, user 0’s keys are generated and installed early in the boot
process. By the time the on-post-fs phase of init
completes, the Keymaster must be ready to handle requests. On Nexus devices,
this is handled by having a script block:

Note: All encryption is based on AES-256 in
XTS mode. Due to the way XTS is defined, it needs two 256-bit keys; so in
effect, both CE and DE keys are 512-bit keys.

Encryption policy

Ext4 encryption applies the encryption policy at the directory level. When a
device’s userdata partition is first created, the basic structures
and policies are applied by the init scripts. These scripts will
trigger the creation of the first user’s (user 0’s) CE and DE keys as well as
define which directories are to be encrypted with these keys. When additional
users and profiles are created, the necessary additional keys are generated and
stored in the keystore; their credential and devices storage locations are
created and the encryption policy links these keys to those directories.

In the current AOSP implementation, the encryption policy is hardcoded into this
location:

/system/extras/ext4_utils/ext4_crypt_init_extensions.cpp

It is possible to add exceptions in this file to prevent certain directories
from being encrypted at all, by adding to the directories_to_exclude
list. If modifications of this sort are made then the device
manufacturer should include
SELinux policies that only grant access to the
applications that need to use the unencrypted directory. This should exclude all
untrusted applications.

The only known acceptable use case for this is in support of legacy OTA
capabilities.

Supporting Direct Boot in system applications

Making applications Direct Boot aware

To facilitate rapid migration of system apps, there are two new attributes that
can be set at the application level. The
defaultToDeviceProtectedStorage attribute is available only to
system apps. The directBootAware attribute is available to all.

The directBootAware attribute at the application level is shorthand for marking
all components in the app as being encryption aware.

The defaultToDeviceProtectedStorage attribute redirects the default
app storage location to point at DE storage instead of pointing at CE storage.
System apps using this flag must carefully audit all data stored in the default
location, and change the paths of sensitive data to use CE storage. Device
manufactures using this option should carefully inspect the data that they are
storing to ensure that it contains no personal information.

When running in this mode, the following System APIs are
available to explicitly manage a Context backed by CE storage when needed, which
are equivalent to their Device Protected counterparts.

Context.createCredentialProtectedStorageContext()

Context.isCredentialProtectedStorage()

Supporting multiple users

Each user in a multi-user environment gets a separate encryption key. Every user
gets two keys: a DE and a CE key. User 0 must log into the device first as it is
a special user. This is pertinent for Device
Administration uses.

Crypto-aware applications interact across users in this manner:
INTERACT_ACROSS_USERS and INTERACT_ACROSS_USERS_FULL
allow an application to act across all the users on the device. However, those
apps will be able to access only CE-encrypted directories for users that are
already unlocked.

An application may be able to interact freely across the DE areas, but one user
unlocked does not mean that all the users on the device are unlocked. The
application should check this status before trying to access these areas.

Each work profile user ID also gets two keys: DE and CE. When the work challenge
is met, the profile user is unlocked and the Keymaster (in TEE) can provide the
profile’s TEE key.

Handling updates

The recovery partition is unable to access the DE-protected storage on the
userdata partition. Devices implementing FBE are strongly recommended to support
OTA using A/B system updates. As
the OTA can be applied during normal operation there is no need for recovery to
access data on the encrypted drive.

When using a legacy OTA solution, which requires recovery to access the OTA file
on the userdata partition:

Create a directory within the top-level directory to hold OTA packages.

Add an SELinux rule and file contexts to control access to this folder and
it contents. Only the process or applications receiving OTA updates should be
able to read and write to this folder. No other application or process should
have access to this folder.

Validation

To ensure the implemented version of the feature works as intended, employ the
many
CTS encryption tests.

Once the kernel builds for your board, also build for x86 and run under QEMU in
order to test with
xfstest by using:

kvm-xfstests -c encrypt -g auto

In addition, device manufacturers may perform these manual tests. On a device
with FBE enabled:

Check that ro.crypto.state exists

Ensure ro.crypto.state is encrypted

Check that ro.crypto.type exists

Ensure ro.crypto.type is set to file

Additionally, testers can boot a userdebug instance with a lockscreen set on the
primary user. Then adb shell into the device and use
su to become root. Make sure /data/data contains
encrypted filenames; if it does not, something is wrong.

AOSP implementation details

This section provides details on the AOSP implementation and describes how
file-based encryption works. It should not be necessary for device manufacturers
to make any changes here to use FBE and Direct Boot on their devices.

ext4 encryption

The AOSP implementation uses ext4 encryption in kernel and is configured to:

Encrypt file contents with AES-256 in XTS mode

Encrypt file names with AES-256 in CBC-CTS mode

Key derivation

Disk encryption keys, which are 512-bit AES-XTS keys, are stored encrypted
by another key (a 256-bit AES-GCM key) held in the TEE. To use this TEE key,
three requirements must be met:

The auth token

The stretched credential

The “secdiscardable hash”

The auth token is a cryptographically authenticated token generated by
Gatekeeper
when a user successfully logs in. The TEE will refuse to use the key unless the
correct auth token is supplied. If the user has no credential, then no auth
token is used nor needed.

The stretched credential is the user credential after salting and
stretching with the scrypt algorithm. The credential is actually
hashed once in the lock settings service before being passed to
vold for passing to scrypt. This is cryptographically
bound to the key in the TEE with all the guarantees that apply to
KM_TAG_APPLICATION_ID. If the user has no credential, then no
stretched credential is used nor needed.

The secdiscardable hash is a 512-bit hash of a random 16 KB file
stored alongside other information used to reconstruct the key, such as the
seed. This file is securely deleted when the key is deleted, or it is encrypted
in a new way; this added protection ensures an attacker must recover every bit
of this securely deleted file to recover the key. This is cryptographically
bound to the key in the TEE with all the guarantees that apply to
KM_TAG_APPLICATION_ID. See the Keystore
Implementer's Reference.